The best argument for getting an index fund is the expense ratio; not broad versus narrow. Managed mutual funds have higher expense ratios because of the broker's salary. Private trading instead of buy and hold will similarly cost you more because of the transaction cost. To justify their transactions, a broker doesn't just have to beat the market, but to beat the market by a large enough swing to justify those extra costs. Because of the number of brokers out there, even if one has consistently beaten the market, it is impossible to determine whether that is due to skill or luck for any given broker. Large domestic index funds will generally have the lowest expense ratios.